Add tiny detail on PORT

[dejavu1987]

I had to go into the source to find out PORT was env.PORT.

As this was not clear in the documentation, this tiny detail might be helpful to users.

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[nvh95]

@all-contributors please add @dejavu1987 for docs

[allcontributors[bot]]

@nvh95

I've put up a pull request to add @dejavu1987! :tada: